1. Field of the Invention
The present invention relates to catalysts for the purification of exhaust gas and a method of manufacturing thereof; and more particularly relates to catalysts for the purification of exhaust gas having an excellent low temperature activity thereby being capable of efficiently purifying hydrocarbon (hereinafter referred to as "HC"), carbon monoxide (hereinafter referred to as "CO") and nitrogen oxide (hereinafter referred to as "NO.sub.x ") in exhaust gas discharged from an internal-combustion engine for automobiles and the like at low temperature as well as excellent catalytic activity under all exhaust gas composition atmospheres such as an oxidizing atmosphere, reducing atmosphere, theoretical air-fuel ratio atmosphere and the like, and a method of manufacturing such catalysts.
2. Description of the Related Art
Conventionally, the low temperature period from the time just after the start-up of an internal-combustion engine to the time when the catalytic temperature reaches a reaction temperature for the purification of exhaust gas, is not sufficient for purifying exhaust gas. Thus, the development of a catalyst which has an ability to purify exhaust gas from a low temperature is desirable.
Catalyst for the purification of exhaust gas are disclosed in Japanese Patent Application Publications No. 58-20307, Japanese Patent Application Opened Nos. 5-305236 and 6-378 and so on.
The catalyst for the purification of exhaust gas described in Japanese Patent Application Publication No. 58-20307 loads a composition consisting of platinum, rhodium and cerium with a refractory carrier. This catalyst essentially has a structure loading a platinum group elements such as platinum, palladium and rhodium on alumina, cerium oxide and the like and coating it onto a monolithic carrier.
Japanese Patent Application Opened No. 5-305236 discloses a catalyst for the purification of exhaust gas prepared by mixing and dispersing hexaaluminate which includes no noble metals into noble metal-loaded alumina such as platinum-, rhodium-, palladium-loaded alumina and the like. This catalyst is essentially a hexaaluminate composition having a specific composition mixed with alumina loading at least one noble metal selected from the group consisting of platinum, rhodium and palladium at a specific weight ratio.
Japanese Patent Application Opened No. 6-378 proposes a catalyst for the purification of exhaust gas incorporating, as a catalyst component, at least one of platinum and palladium and at least one metal oxide selected from the group consisting of potassium, cesium, strontium and barium as a basic element on activated alumina and cerium oxide. That is, said catalyst comprises a combination of at least one basic element selected from the group consisting of potassium compound, cesium compound, strontium compound and barium compound in addition to the platinum element, active alumina, cerium oxide and the like hitherto used as catalyst components.
The aforementioned catalysts use a great amount of noble metals, but these noble metals are not abundant and are expensive. Therefore, it is desired to develop a high performance catalyst with less of a noble metal content used as a three-way catalyst capable of purifying exhaust gas. However, in the case of a lower noble metal content, the purification ability of the catalyst is lowered when the exhaust gas atmosphere becomes a reducing atmosphere. Also, low temperature activity and purification performance are decreased.
In a reducing atmosphere in which oxygen concentration is not sufficient for purification of hydrocarbon, a part of hydrocarbon is not purified and the unpurified hydrocarbon is strongly adsorbed onto the noble metal to lower its activity or to reduce active point. As a result, the purification performance is considerably lowered. Particularly, in case of a lowered content of palladium, the above deleterious effects appear conspicuously, resulting in the purification performance being further lowered.
Moreover, a catalyst for the purification of exhaust gas should purify the exhaust gas of an automobile having a widely varying composition within a wide temperature range from a low temperature region to a high temperature region at a high purification ratio. However, in purification of exhaust gas by palladium, the purification performance for hydrocarbons is considerably lowered at low temperatures just after starting an engine, or in an atmosphere of high hydrocarbon concentration and low oxygen concentration, while in an atmosphere of high oxygen concentration and low hydrocarbon concentration, nitrogen oxide is hard to adsorb since oxygen adsorbs on palladium, resulting in the purification performance for the nitrogen oxide being greatly lowered.